linux-kernel - Re: [PATCH] trace: reset sleep/block start time on task switch

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Message-ID: <4F1F26CC.4040109@fb.com>
Date:	Tue, 24 Jan 2012 13:46:52 -0800
From:	Arun Sharma <asharma@...com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	<linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Andrew Vagin <avagin@...nvz.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] trace: reset sleep/block start time on task switch

On 1/24/12 6:27 AM, Peter Zijlstra wrote:

> Bah, you're right. Also yes your proposal is too intrusive, but that can
> be fixed, I actually did, but then I noticed its broken too, it doesn't
> matter if the schedule that schedules a task back in preempted another
> task or not, what matters is if the task we're scheduling back in was
> itself preempted or recently woken. And we simply don't know.

Yes - we'd need an extra bit in the task_struct to do this right.

> I'm tempted to revert 1ac9bc69 for now, userspace will simply have to
> correlate trace_sched_switch() and trace_sched_stat_{sleep,blocked}(),
> which shouldn't be too hard.

We tried it and it didn't work very well. Especially when used with perf 
record -g. There are too many uninteresting trace_sched_switch() events.

Other possibilites: make a copy of {sleep,block}_start somewhere else in 
the perf_events subsystem in the sleep/wakeup path and leave 
sched_statistics untouched.

  -Arun


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